We have used paramagnetic probes to monitor the orientations of both myosin and actin in solution and in muscle fibers. Spectra obtained from these studies can then determine which portions of these proteins undergo changes during the generation of force produced by muscle contraction. Theories of muscle contraction have envisioned a cyclic interaction in which a myosin "cross-bridge" attaches to an actin filament and then causes, through some "powerstroke," the shortening of the sarcomere. The popular "rowing oar" model for muscle contraction invokes a change in orientation of myosin during the powerstroke. The field of motor proteins has recently made a great leap forward with the publication of the structures of both actin and myosin. A model of the actomyosin complex derived from these structures has led to a model of the force generating cycle. In this hypothesis, the movement of myosin is similar to the domain shifts seen in a number of other proteins, making it very attractive. However, there is little direct experimental evidence to support the existence of the proposed changes in structure. Our recent work with spin probes supports some aspects of this hypothesis. To be useful, our results obtained with spin probes must be interpreted in term of the structures. Modeling of the spin probes into the known structures using the MidasPlus modeling system and Silicon Graphics workstations available at the the UCSF Computer Graphics Lab provides additional information on how they report conformational changes in the proteins. The structure of the myosin head consists of a large distal region that contains the sites for interaction with both actin and nucleotides. This region is connected to the junction with the thick filament by a long tapering neck region. Rayment and coworkers proposed that the distal region attaches rigidly to actin followed by a change in orientation of the neck region. Thus the neck region would constitute the "rowing oar" proposed by earlier theories. The movement of the neck is thought to be propelled by the opening of a pocket that binds nucleotides. Binding ATP would close this pocket, followed by binding of actin and release of nucleotides which would open the pocket and drive the rotation of the neck region.